In my project, I need to have highly precise time syncronisation between two modules (in µs). I am not supposed to use any wires in between. So I was looking for wireless devices like RF or IR modules. I need timley delivery of message in micro second. I am not worried about so called latency time since I can start my communication in advance. But after that, any data (few bits) sent should happen in few hundread nanoseconds.

I don't have any experience in WLAN or IRDA. But I beleive IRDA and WLAN are meant for accurate data and not timely delivery. Viz. it will take it's own time to send the data (even the speed is in Mbps) and I don't have any chance to interfere at low level drivers in that protocol stack.

Some further detail is necessary. If you send data to a receiver, it can pass with nothing more elaborate than a shared carrier frequency. I have been around suppressed carrier data links, where it was necessary to enable the carrier in order for one of the receiving stations to get synched up.

Ordinarily, a radio receiver just waits for input. When a carrier is detected, the squelch circuit indicates activity. That can be the wake-up for you data receiver.

Here, two modules kept apart need to do "some activity"; almost simultaniously. The max differnce could be 5 µs. So to initiate this ativity, one module has to inform that it has started. I don't want "wake up" time by a squech ciruit be in picture.

So, if the carrier has already been detected and some data communication is happeneing already, say a bit stream of "1010" continuosly. And now if I trasmit say "1001" indicating the sync. Will reception take place in 1µ if the datarate for the wireless module is 4Mbps?

4 bits at a 4 meg rate should go in 1 uS. The only delay is the speed of light delay between the transmitter and receiver.

That would be accurate for a direct RF link. Dealing with WLAN, your data gets placed in packets and transmitted through a larger network. I am not at all sure that the degree of packet transmission latency can be predicted. There will be the time necessary to form the packet for transmission and unpacking it at the receive end.

Using an IR link avoids packaging the data into packets, and gives a predictable data rate, if limited range. IRDA does support the 4 mbps rate (unless the protocol forces some overhead by doing CRC checks).